The present exemplary embodiment relates to additives/colorants plastics processing, for example an extrusion method and apparatus. It finds particular application in conjunction with a method and apparatus for providing a proportional addition of additive into material during plastics processing, for example the extrusion process, and will be described with particular reference thereto. Applicable extrusion processes include but are not limited to sheet, film and fiber. However, it is to be appreciated that the present exemplary embodiment is also amenable to other plastic applications and processes. For example, it may be applied to situations wherein additives are introduced to molten polymer which comes directly from a reactor, for example a reactor in which melt polymerization is undertaken and/or wherein there is no extruder into which polymer pellets would conventionally be fed.
The plastics melting process uses one or more screws to force plastic beads into a barrel of an extruder heated to the desired melt temperature. The plastic beads melt gradually as they pass through the barrel. The plastics melting process is highly dependent on a consistent feed of bulk materials into the feed throat to maintain consistent output. Consistent output is necessary for consistent finished product properties such as size, strength and appearance. The properties of the finished product are determined by the base materials, tillers, and additives including colorants that are blended before and during the melt phase of the process. Additives such as pigments, dyes, flame retardants and antimicrobial agents are often added to extrusion and injection molding processes to modify the physical and chemical properties of materials making the material more desirable. All plastics melting processes have various means of control which lead to longer or shorter feedback loops to the feeding equipment to manage and maintain correct proportional output.
Bulk materials in the forms of powders, granules, pellets, irregular chunks, and irregular flakes are produced from various processes that are conveyed to the extruder or injection molding machine. These materials are typically blended and held in a hopper or material line of various sizes before being conveyed to the feed throat of the machine. Typically these materials reed into the process by gravity to the screw which, in this section of the extruder, acts as a conveyor to sections of the process where heat and pressure are applied to melt and mix the materials. In those cases where the material does not feed consistently by gravity, especially where flakes or irregular shapes are present, mechanical means such as crammer feeders, or stuffers are used in an attempt to manage consistent feeding of materials.
Dry powder, pellet and liquid forms of additives may be added to the stream of bulk materials before the introduction to the screw. The material feeders in this case are auger feeders, disk or wheel feeders, belts, vibratory feeders, simple timers and pumps in the case of liquid additives. By adding these materials to the material stream before introduction to the screw, an amount of material is created which may have had correct proportion upon delivery, but is committed to the process regardless of the proportion of the material produced upon output of the process.
Another method for introducing additives into a material is to inject liquid additive directly into the barrel of the apparatus. However the start up protocol involved with this method is complex and recovery from problems that arise is extremely difficult. Moreover, the amount of scrap produced in this method is greatly increased as a result of the foregoing issues.
The final form of the material processed by the one or more screws is a viscous liquid melt which will be further processed by the machine to produce the final shape or form. This final shape ranges from the manufacture of fibers, strands and extruded shapes, to injection molded products of various forms. A large amount of material, time and energy is committed to the process to produce this melt. A natural efficiency exists when valuable additives are added to the process at the last possible moment while maintaining the potential to thoroughly mix, disperse, or saturate the polymer stream with the additive in precise proportions to the materials and in correct proportion to the output of the process.
Moreover, developing feed stocks of consistent sizes, shapes, melt index, density and color is becoming more difficult for manufacturers as raw materials become more expensive and feed stocks become more valuable in a world of higher and higher value of oil products. The requirement for material property modifiers further complicates the sourcing and most frequently adds substantial cost to the materials already in short supply. The introduction of additives and more importantly the ability to deliver liquid additives into the process is preferred to using higher specification materials where they are not needed. Liquid additives allow the choice of inclusion and proportion at the process, to minimize cost. The material input into the process is now the result of the requirements of the finished product rather than the availability of higher specified materials, which significantly reduces cost. Liquid additive injection further allows very short control loops to the output of the process since liquids may be pumped at elevated temperatures and pressures.
Inconsistent feeding of hulk materials has also led to inconsistencies and inefficiencies in the process, changing the size of the extruded product and its variable physical and appearance properties. Introduction at the latest point of the process assures correct proportion relative to process conditions (elevated pressures and temperatures) that a solid feeder cannot. Further sophisticated pumping systems allow the amount of in process mixed material to be very small, leading to less material involved in color changes and more rapid reaction to changes in the process in real time. As process outputs change, the introduction of the additive changes proportionally nearly simultaneously, thus preventing problems with over dosing or under dosing of additive.
The following references, the disclosures of which are incorporated herein by reference in their entireties, variously relate to colored plastics molding devices and systems. U.S. Pat. No. 5,486,327 teaches of an arrangement for introducing a liquid additive to the barrel of a screw-type mechanism for converting solid material to molten material. However, this method introduces the additive close to the feed throat, thus leading to inconsistent input variables with regrind and density variation associated with less expensive raw materials.
U.S. Pat. No. 5,439,623 provides a method for controlling the introduction of additives, in the form of a plurality of strands, to a cutter. The method uses the feed rate, velocity and delta position for each strand per feed interval in order to obtain the desired amount of additive. However, this method does not use a control logic method to proportionately inject additive according to the output of the extruder.
Thus, there is a need for an additive introduction method that provides consistent, proportional additive into the plastics melting process, while reducing cost and increasing efficiency.
Further scope of the applicability of the present invention will become apparent from the detailed description provided below. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.